方法指针的 RTTI 信息

Question

是否可以获得关于TMethod 的RTTI 信息？

我可以通过

Instance := TObject(Method.Data);

所以我可以获得实例的 RTTI 类型，但我怎样才能获得正确的 TRttiMethod？我想检查使用方法指针传入的方法的属性。

Answer 1

这种方法在理论上可行，并且在实践中会有很好的改变，但有几件事可能会阻止您获得TRttiMethod。

• TMethod 记录显示 Data: Pointer，而不是 TObject。这意味着可能有除TObject 之外的其他东西作为Data！这是一个严重的问题，因为如果 Data 不是 TObject，那么尝试从中提取 RTTI 将导致运行时错误。
• 并非所有方法都有 RTTI。默认情况下，私有区域中的方法没有 RTTI，也可以使用{$RTTI} 停止为公共或已发布成员生成 RTTI。

这两个问题对于我们在 Delphi 中的通常类型的事件实现来说不是问题（在 Object Inspector 中双击事件的名称并填写代码），但我不认为你在谈论“vanila”实现。没有多少人会用属性来装饰默认事件处理程序！

演示以上所有内容的代码：

program Project15;

{$APPTYPE CONSOLE}

uses
  SysUtils, RTTI;

type
  // Closure/Event type
  TEventType = procedure of object;

  // An object that has a method compatible with the declaration above
  TImplementation = class
  private
    procedure PrivateImplementation;
  public
    procedure HasRtti;

    procedure GetPrivateImpEvent(out Ev:TEventType);
  end;

  TRecord = record
    procedure RecordProc;
  end;

  // an object that has a compatible method but provides no RTTI
  {$RTTI EXPLICIT METHODS([])}
  TNoRttiImplementation = class
  public
    procedure NoRttiAvailable;
  end;

procedure TImplementation.GetPrivateImpEvent(out Ev:TEventType);
begin
  Ev := PrivateImplementation;
end;

procedure TImplementation.HasRtti;
begin
  WriteLn('HasRtti');
end;

procedure TNoRttiImplementation.NoRttiAvailable;
begin
  WriteLn('No RTTI Available');
end;

procedure TRecord.RecordProc;
begin
  WriteLn('This is written from TRecord.RecordProc');
end;

procedure TImplementation.PrivateImplementation;
begin
  WriteLn('PrivateImplementation');
end;

procedure TotalyFakeImplementation(Instance:Pointer);
begin
  WriteLn('Totaly fake implementation, TMethod.Data is nil');
end;

procedure SomethingAboutMethod(X: TEventType);
var Ctx: TRttiContext;
    Typ: TRttiType;
    Method: TRttiMethod;
    Found: Boolean;
begin
  WriteLn('Invoke the method to prove it works:');
  X;
  // Try extract information about the event
  Ctx := TRttiContext.Create;
  try
    Typ := Ctx.GetType(TObject(TMethod(X).Data).ClassType);
    Found := False;
    for Method in Typ.GetMethods do
      if Method.CodeAddress = TMethod(X).Code then
      begin
        // Got the Method!
        WriteLn('Found method: ' + Typ.Name + '.' + Method.Name);
        Found := True;
      end;
    if not Found then
      WriteLn('Method not found.');
  finally Ctx.Free;
  end;
end;

var Ev: TEventType;
    R: TRecord;

begin
  try
    try
      WriteLn('First test, using a method that has RTTI available:');
      SomethingAboutMethod(TImplementation.Create.HasRtti);
      WriteLn;

      WriteLn('Second test, using a method that has NO rtti available:');
      SomethingAboutMethod(TNoRttiImplementation.Create.NoRttiAvailable);
      WriteLn;

      WriteLn('Third test, private method, default settings:');
      TImplementation.Create.GetPrivateImpEvent(Ev);
      SomethingAboutMethod(Ev);
      WriteLn;

      WriteLn('Assign event handler using handler from a record');
      try
        SomethingAboutMethod(R.RecordProc);
      except on E:Exception do WriteLn(E.Message);
      end;
      WriteLn;

      WriteLn('Assign event handler using static procedure');
      try
        TMethod(Ev).Data := nil;
        TMethod(Ev).Code := @TotalyFakeImplementation;
        SomethingAboutMethod(Ev);
      except on E:Exception do WriteLn(E.Message);
      end;
      WriteLn;

    except
      on E: Exception do Writeln(E.ClassName, ': ', E.Message);
    end;
  finally ReadLn;
  end;
end.